[css-shadow-parts] Add support for multiple names in ::part(), as discussed with Fergal.

Author: tabatkins

css-shadow-parts-1/Overview.bs

@@ -249,13 +249,13 @@ allows you to select elements that have been exposed via a <{html-global/part}>
 The syntax is:
 
 <pre class=prod>
-    ::part() = ::part( <<ident>> )
+    ::part() = ::part( <<ident>>+ )
 </pre>
 
 The ''::part()'' pseudo-element only matches anything
 when the <a>originating element</a> is a <a>shadow host</a>.
 If the <a>originating element's</a> <a>shadow root's</a> <a>part element map</a>
-[=map/contains=] the specified <<ident>>,
+[=map/contains=] all of the specified <<ident>>s,
 ''::part()'' matches the element or elements keyed to that <<ident>>.
 Otherwise, it matches nothing.
 
@@ -268,15 +268,30 @@ Otherwise, it matches nothing.
     ''#the-button::part(label)''.
 </div>
 
-The <a>shadow-part pseudo-elements</a> can take additional pseudo-classes after them,
+<div class="example">
+	Part names act similarly to classes:
+	multiple elements can have the same part name,
+	and a single element can have multiple part names.
+
+	A tabstrip control might have multiple elements
+	with <code>part="tab"</code>,
+	all of which are selected by ''::part(tab)''.
+
+	If a single tab is active at a time,
+	it can be specially indicated with <code>part="tab active"</code>
+	and then selected by ''::part(tab active)''
+	(or ''::part(active tab)'', as order doesn't matter).
+</div>
+
+The ''::part()'' pseudo-element can take additional pseudo-classes after it,
 such as ''x-button::part(label):hover'',
-but never match the <a>structural pseudo-classes</a>
+but never matches the <a>structural pseudo-classes</a>
 or any other pseudo-classes that match based on tree information
 rather than local element information.
 
-The <a>shadow-part pseudo-elements</a> also can take additional pseudo-elements after them,
+The ''::part()'' pseudo-element can also take additional pseudo-elements after it,
 such as ''x-button::part(label)::before'',
-but never match additional <a>shadow-part pseudo-elements</a>.
+but never match additional ''::part()''s.
 
 <div class=example>
     For example,